The invention pertains to negative thermal expansion materials, devices made therefrom, and methods of making the materials. More particularly, the present invention concerns compositions including zirconium phosphate tungstates, which can be used to make substrates for athermalized optical fiber reflective grating devices.
Negative thermal expansion (contraction with increasing temperature) is an unusual and potentially useful property for a solid material, and very few crystalline materials possess strongly negative expansions over an extended temperature range. Materials that exhibit a negative expansion due to extensive microcracking, by virtue of a negative coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) along at least one crystallographic axis and a substantially different CTE along at least one other axis, include some lithium aluminosilicates, xe2x80x9cNZPsxe2x80x9d (compounds with crystal structures similar to that of NaZr2P3O12) , and Ta2O5-WO3 compounds. On the other hand, materials having negative mean lattice expansions that do not require microcracking for negative bulk ceramic CTEs are even more limited, and include certain synthetic alkali-free zeolites having low aluminum contents, ZrW2O8, HfW2O8, ZrV2-xPxO7 (above about 100xc2x0 C.), and NbZrP3O12 (an NZP type compound).
Of the compounds that have negative mean lattice expansions, zeolite expansions from 25 to 100xc2x0 C. are in the range xe2x88x9220xc3x9710xe2x88x927xc2x0 C.xe2x88x921 to xe2x88x9240xc3x9710xe2x88x927xc2x0 C.xe2x88x921, but have the disadvantages of being dependent upon the amount of adsorbed water in the zeolite, and their CTE curves can exhibit considerable hysteresis. The CTE of NbZrP3O12 below 100xc2x0 C. is about xe2x88x9227xc3x9710xe2x88x927xc2x0 C.xe2x88x921, while that of ZrW2O8 and HfW2O8 is about xe2x88x9290xc3x9710xe2x88x927xc2x0 C.xe2x88x921. Thus, ZrW2O8 and HfW2O8 are presently unique as materials that display a CTE more negative than xe2x88x9240xc3x9710xe2x88x927xc2x0 C.xe2x88x921 without microcracking.
Martinek and Hummel (1960, J. Amer. Ceram. Soc., 53, 159-161) first reported the existence of Zr2P2WO12 in their study of the phase relations in the ZrO2-WO3-P2O5 system at 1125xc2x0 C. An XRD powder pattern was presented for this new compound, which reportedly has a melting point above 1750xc2x0 C., although extensive volatilization occurs in air at 1600xc2x0 C. Synthesis of Zr2P2WO12 was achieved by calcining a mixture of hydrous zirconium carbonate, tungstic acid, and mono-hydrogen ammonium phosphate.
Tsvigunov and Sirotinkin (1990, Russ. Jour.of Inorg. Chem., 35, 1740) subsequently reported a more complete and precise powder XRD pattern for this compound, which they synthesized from a mixture of ZrO2, WO3, and NH4H2PO4. Evans et al. (1995, Jour. Solid State Chem., 120, 101-104) have shown that the structure of Zr2P2WO12 (also referred to as Zr2(WO4)(PO4)2) is comprised of ZrO6 octahedra sharing corners with WO4 and PO4 tetrahedra. Those workers report that dilatometric and variable temperature X-ray diffractometry studies indicate that Zr2P2WO12 exhibits a negative thermal expansion over a broad temperature range. More recently, Evans et al. (1997, Journal Solid State Chem., 133, 580-83) have reported that Zr2P2WO12 has a mean lattice CTE of about xe2x88x9230xc3x9710xe2x88x927xc2x0 C.xe2x88x921.
The Zr2P2WO12 bodies synthesized according to the methods reported in the above literature have porosities greater than about 25%, typically greater than 30%. Such high porosity bodies generally are not useful for industrial applications. Thus, it would be useful to provide a composition having a low, preferably a negative thermal expansion, comprised of Zr2P2WO12, or analogues thereof in which Hf is fully or partially substituted for Zr, having a porosity less than about 20%, preferably less than about 10%, and more preferably less than about 5%.
Bodies having a highly negative CTE, such as xe2x88x9230 to xe2x88x92100xc3x9710xe2x88x927xc2x0 C.xe2x88x921, can find use as substrates for athermalization of fiber Bragg gratings (FBGs). In the latter application, a FBG is mounted in tension on the negative expansion substrate. Applications of FBGs include passive wavelength division multiplexing and filtering in dense WDM systems, as well as distributed fiber sensors for smart systems to monitor bridges, structures, and highways.
For such applications, variation of the center wavelength of fiber Bragg gratings (FBGs) with respect to temperature, due to thermal expansion of the fiber and variation of the refractive index of the glass, must be minimized. For example, at a Bragg wavelength of 1550 nm, thermal variation of xcexB is expected to be 0.012 nm/xc2x0 C., whereas a value less than 0.002 nm/xc2x0 C. is desired. Variation in xcexB with temperature can be reduced to well below 0.002 nm/xc2x0 C. by mounting the FBG in tension on a substrate having a negative thermal expansion of about xe2x88x9270 to xe2x88x9285xc3x9710xe2x88x927xc2x0 C.xe2x88x921 within that range also xe2x88x9270 to xe2x88x9280xc3x9710xe2x88x927xc2x0 C.xe2x88x921, xe2x88x9275 to xe2x88x9282xc3x9710xe2x88x927xc2x0 C.xe2x88x921. The reduction in tension with increasing temperature associated with the contraction of the substrate partially or entirely offsets the contribution to increased optical path length resulting from the thermal expansion and change in refractive index of the glass.
xcex2-eucryptite based ceramics formed by controlled devitrification of sintered lithium aluminosilicate glass are being studied as FBG substrates and are disclosed in international patent application no. PCT/US/13062, Beall et al., entitled, xe2x80x9cAthermal Optical Device.xe2x80x9d The attainment of CTEs of xe2x88x9270 to xe2x88x9285xc3x9710xe2x88x927xc2x0 C.xe2x88x921 in xcex2-eucryptite bodies requires extensive microcracking; an unmicrocracked xcex2-eucryptite exhibits a CTE near xe2x88x925xc3x9710xe2x88x927xc2x0 C.xe2x88x921. This microcracking results from internal stresses associated with the large difference in CTE along the c and a axes of the crystals (approximately xe2x88x92176 and +78xc3x9710xe2x88x927xc2x0 C.xe2x88x921, respectively), coupled with the coarse grain size of the crystals.
U.S. Pat. No. 5,694,503, issued to Fleming et al., discloses using the negative coefficient of thermal expansion material ZrW2O8 to form substrates for temperature compensated fiber Bragg gratings. Since the coefficient of thermal expansion of ZrW2O8 may be too negative to provide temperature compensation for Bragg gratings, the Fleming et al. patent suggests mixing ZrW2O8 with a positive coefficient of thermal expansion material such as alumina, silica, zirconia, magnesia, calcia, or yttria in an amount to raise the coefficient of thermal expansion.
The mixtures of ZrW2O8 with positive coefficient of thermal expansion materials suggested in the Fleming et al. patent, however, have several disadvantages. Large relative differences in the thermal expansion coefficients of ZrW2O8 and the positive CTE materials can cause microcracking in the composite material upon heating and cooling of the material. Such microcracking can result in hysteresis in the thermal expansion curve or dimensional change of the body with changes in humidity, characteristics that are undesirable in a fiber Bragg grating substrate. Furthermore, many of the positive CTE components recommended in the Fleming et al. patent react with the ZrW2O8 during sintering to form copious amounts of liquid. Such reactions and liquid formation tend to cause the body to slump during firing. Alternatively, some of the positive CTE components recommended in the Fleming et al. patent react with the ZrW2O8 to form other high CTE crystalline phases so that the ceramic body does not have the desired strongly negative CTE after firing. In addition, ceramics comprised of ZrW2O8 and ZrO2 undergo a length change having an absolute value greater than 500 parts per million over 700 hours at 85% relative humidity and 85xc2x0 C., which is undesirably large.
The presence of microcracking in a FBG substrate requires that the fiber/substrate package be hermetically sealed to prevent dimensional drift of the substrate due to opening and closing of the microcracks resulting from variations in humidity. Hermetic sealing adds significantly to the cost of the assembly, and the reliability of the device becomes dependent upon long-term reliability of the hermetic seal.
Thus, it would be desirable to provide an unmicrocracked material having a porosity less than about 25%, preferably less than about 10% and more preferably less than about 5%. Further, there is a need for a body having a CTE of about xe2x88x9270xc3x9710xe2x88x927xc2x0 C.xe2x88x921 to xe2x88x9285xc3x9710xe2x88x927xc2x0 C.xe2x88x921 to provide temperature compensation for the gratings of current interest which could be used to make FBG substrates because hermetic sealing would not be required for long-term stability. In addition, it would be desirable to provide a material that has a length change having an absolute value less than 500 ppm over 700 hours at 85xc2x0 C. and 85% relative humidity.
The present invention provides a low-porosity body containing at least one phase having a negative thermal expansion, a method of making the phase, and devices made from the phase. In one embodiment, the body is comprised of the compound Zr2P2WO12 which exhibits a room-temperature CTE of about xe2x88x9240xc3x9710xe2x88x927xc2x0 C.xe2x88x921, and a CTE from about 25 to 800xc2x0 C. of about xe2x88x9225xc3x9710xe2x88x927xc2x0 C.xe2x88x921. Hafnium may be partially or entirely substituted for zirconium. The body further comprises a crystalline or non-crystalline oxide phase, which may include a glassy phase, which contains a metal selected from the group consisting of alkaline earth metals, alkali metals, manganese, iron, cobalt, copper, zinc, aluminum, gallium, and bismuth. The oxide phase may also contain one or more of the metals selected from the group zirconium, tungsten and phosphorous.
In another embodiment, this invention comprises a ceramic body comprised of two negative CTE phases, preferably wherein at least one of the phases has a room temperature CTE more negative than xe2x88x9250xc3x9710xe2x88x927Cxe2x88x921. In an exemplary embodiment, one phase has the composition M2B3O12 where M is selected from the group including aluminum, scandium, indium, yttrium, the lanthanide metals, zirconium, and hafnium, and where B is selected from the group consisting of tungsten, molybdenum, and phosphorus, and where M and B are selected such that the compound M2B3O12 has a negative CTE, and comprising a second phase of the composition AX2O8, where A is selected from the group consisting of zirconium and hafnium, and X is selected from the group consisting of tungsten and molybdenum.
For example, the ceramic body may comprise a mixture of ZrW2O8 and Zr2P2WO12, wherein hafnium may be partially or fully substituted for zirconium in Zr2P2WO12 and ZrW2O8. In a preferred embodiment, the ceramic body is unmicrocracked. The ceramic body may further include a crystalline or non-crystalline oxide phase, which may include a glassy phase, which contains a metal selected from the group consisting of alkaline earth metals, alkali metals, lanthanum group metals, niobium, titanium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, scandium, aluminum, gallium, and bismuth. The oxide phase may also contain one or more of the metals selected from the group zirconium, tungsten and phosphorous.
The invention also includes a method of making ceramic bodies of the present invention comprising mixing together powders of Zr2P2WO12 or precursor powders of Zr2P2WO12 or analogues of these powders in which Hf is substituted for Zr, or mixtures thereof, and at least one oxide or oxide precursor of metals selected from the group consisting of alkaline earth metals, alkali metals, manganese, iron, cobalt, copper, zinc, aluminum, gallium, and bismuth.
Another embodiment of this invention involves a method of raising and making the negative thermal expansion of a ceramic body less negative comprising a phase having a room temperature coefficient of thermal expansion more negative than 50xc3x9710xe2x88x927xc2x0 C.xe2x88x921 comprising mixing together the first phase with at least a second phase or precursors of the first phase and at least the second phase, the second phase having a negative thermal expansion less negative than xe2x88x9250xc3x9710xe2x88x927xc2x0 C.xe2x88x921. A body made according to this method preferably will not exhibit microcracking. Alternatively, the method includes forming a ceramic body comprising two negative CTE phases, preferably wherein the room temperature CTE of one of the phases is more negative than xe2x88x9250xc3x9710xe2x88x927Cxe2x88x921 and the other phase is less negative than xe2x88x9250xc3x9710xe2x88x927Cxe2x88x921, to provide a body having a CTE less negative than xe2x88x9250xc3x9710xe2x88x927Cxe2x88x921.
In an exemplary embodiment, one phase has the composition M2B3O12 where M is selected from the group including aluminum, scandium, indium, yttrium, the lanthanide metals, zirconium, and hafnium, and where B is selected from the group consisting of tungsten, molybdenum, and phosphorus, and where M and B are selected such that the compound M2B3O12 has a negative CTE, and comprising a second phase of the composition AX2O8, where A is selected from the group consisting of zirconium and hafnium, and X is selected from the group consisting of tungsten and molybdenum. In another exemplary embodiment, the method comprises mixing ZrW2O8 or precursors of ZrW2O8, or analogues of these materials in which Hf is substituted for Zr, with Zr2P2WO12 or precursor powders of Zr2P2WO12 or analogues of these powders in which Hf is substituted for Zr. Optionally, these may also be mixed with at least one oxide or oxide precursor of metals selected from the group consisting of alkaline earth metals, alkali metals, manganese, iron, cobalt, copper, zinc, aluminum, gallium, and bismuth. The mixed powders are consolidated together using a ceramic forming method and heated to sinter the ceramic body. Preferably, the heating occurs at a temperature of about 1050xc2x0 C. to 1300xc2x0 C., more preferably 1120xc2x0 C. to 1230xc2x0 C. for about 1 minute to 10 hours. When the ZrW2O8 phase is desired to be present in the ceramic body, preferably heating occurs at a temperature of about 1150xc2x0 C. to 1230xc2x0 C.
Another aspect of the invention involves an optical device comprising a negative expansion substrate having a composition comprising two negative CTE phases, preferably wherein at least one of the phases has a room temperature CTE more negative than xe2x88x9250xc3x9710xe2x88x927xc2x0 C.xe2x88x921, and one of the phases has a thermal expansion less negative than xe2x88x9250xc3x9710xe2x88x927xc2x0 C.xe2x88x921. For example, the substrate composition may comprise a mixture of ZrW2O8 and Zr2P2WO12. Hafnium may be partially or fully substituted for zirconium in Zr2P2WO12 and ZrW2O8. In a preferred embodiment, the substrate is unmicrocracked. The substrate may further include a crystalline or non-crystalline oxide phase, which may include a glassy phase, which contains a metal selected from the group consisting of alkaline earth metals, alkali metals, lanthanum group metals, niobium, titanium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, scandium, aluminum, gallium, and bismuth. The oxide phase may also contain one or more of the metals selected from the group zirconium, tungsten and phosphorous.
The device further comprises a thermally sensitive, positive expansion optical component affixed to the substrate. In one embodiment, the optical component is an optical fiber grating and the substrate has a mean linear coefficient of thermal expansion of about xe2x88x9240xc3x9710xe2x88x927xc2x0 C.xe2x88x921 to xe2x88x9285xc3x9710xe2x88x927xc2x0 C.xe2x88x921 over a temperature range of about xe2x88x9240xc2x0 C. to 85xc2x0 C. Another aspect of the invention relates to a negative expansion substrate having a composition comprising Zr2P2WO12 and a crystalline or non-crystalline oxide phase, which may include a glassy phase, which contains a metal selected from the group consisting of alkaline earth metals, alkali metals, manganese, iron, cobalt, copper, zinc, aluminum, gallium, and bismuth. Hafnium may be fully or partially substituted for zirconium. The oxide phase may also contain one or more of the metals selected from the group zirconium, tungsten and phosphorous.
Thus the present invention generally provides a novel ceramic body comprised of phase having a negative CTE, such as Zr2P2WO12 or Hf2P2WO12 or mixtures thereof, which exhibits a negative coefficient of thermal expansion (CTE) at all temperatures from at least as low as 25xc2x0 C. to at least as high as 500xc2x0 C. The mean CTE near room temperature is about xe2x88x9240xc3x9710xe2x88x927xc2x0 C.xe2x88x921, while the mean CTE from 25 to 800xc2x0 C. is about xe2x88x9225xc3x9710xe2x88x927xc2x0 C.xe2x88x921. The Zr2P2WO12 or Hf2P2WO12 phase is stable at all temperatures from at least as low as xe2x88x9250xc2x0 C. to at least as high as 1150xc2x0 C.
Also disclosed is a method for fabricating the sintered ceramic body which, in some embodiments, entails the addition of small amounts of additives which function as sintering aids to powders of Zr2P2WO12 or Hf2P2WO12 or mixtures thereof or their precursors. These additives include the compounds of lithium, sodium, potassium, magnesium, calcium, barium, manganese, iron, copper, and zinc. Aluminum compounds may also be used as sintering aids, but are not as effective. Compounds of rubidium, cesium, and strontium would also likely be effective for densification.
The present invention also includes a ceramic body comprised mainly of the phases ZrW2O8 and Zr2P2WO12 and their hafnium analogues and mixtures thereof, having a mean linear coefficient of thermal expansion of about xe2x88x9240 to xe2x88x9285xc3x9710xe2x88x927xc2x0 C.xe2x88x921 over the temperature range xe2x88x9240xc2x0 C. to +85xc2x0 C. These bodies also exhibit a negative CTE to higher temperatures as well. Preferred embodiments of the invention have less than 10% total porosity, especially less than 5% porosity. In one embodiment, achievement of low porosity is enhanced by the addition of small amounts (0.01 to 5.0 wt %) of certain sintering additives, such as the oxides or oxide-forming compounds of alkali (group IA) metals, alkaline earth (group IIA) metals, manganese, iron, cobalt, nickel, copper, zinc, yttrium, scandium, lanthanide metals, niobium, titanium, aluminum, gallium, and bismuth. Many of these materials have the desirable properties of having excellent dimensional stability at 85xc2x0 C. and 85% relative humidity and possessing no microcracking, and thus exhibit no hysteresis in their thermal expansion curves. Such ceramics are suitable as athermalizing substrates for fiber Bragg gratings.
Additional features and advantages of the invention will be set forth in the description which follows. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description serve to explain the principles of the invention. In the drawings, like reference characters denote similar elements throughout the several views. It is to be understood that various elements of the drawings are not intended to be drawn to scale, but instead are sometimes purposely distorted for the purposes of illustrating the invention.